The present invention generally relates to fiber optic communications and, more particularly, to feedback control of optical beam alignment in a 3-dimensional, all-optical, fiber optical switch.
Fiber optical switches find wide application in communications. Fiber optical switches are increasingly used in the telecommunications industry, where fiber optical switches may be used, for example, in a central office core router of a telecommunications network as cross-connect switches for metro and long haul services.
FIG. 1A shows an optical communication system hierarchy 100 according to the prior art, including long haul and metro telecommunications switching networks, for example, long haul switching network 102 and metro telecommunications switching networks 104 and 106. Optical communication system hierarchy 100 may include nodes—such as nodes 108—that communicate using optical fiber links—such as links 110—between the nodes, typically connected in loops. Optical fibers may be used in the links—such as links 110—as working, protection, add, or drop links, as known in the art, for transmitting signal light beams between nodes—such as nodes 108. A node may be, for example, a telephone exchange, such as public switched telephone network (PSTN) 112 or cellular network 114, shown in FIG. 1A connected, for example, by a synchronous optical network (SONET) network 116. As seen in FIG. 1A, for example, metro telecommunications switching network 104 may be connected via one or more optical links—such as optical links 117—to residential extended digital subscriber line (x-DSL) network 118. Also as seen in FIG. 1A, for example, metro telecommunications switching network 106 may be connected via one or more optical links—such as optical links 119—to internet protocol (IP) router 120, connecting asynchronous transfer mode (ATM) switch 122 and Ethernet local area network (LAN) 124 for a regional internet service provider (ISP). Also as seen in FIG. 1A, for example, metro telecommunications switching network 106 may be connected via one or more optical links—such as optical links 125—to a corporate enterprise systems connection (ESCON) network 126, which may comprise a frame relay ESCON fiber channel network or gigabit Ethernet, as known in the art. Each of PSTN 112, cellular network 114, SONET network 116, residential x-DSL network 118, IP router 120, ATM switch 122, Ethernet LAN 124, and ESCON network 126 may connected through an optical cross connect switch to a switching network such as metro telecommunications switching networks 104 and 106.
Referring now to FIG. 1B, an example of a long haul switching network 130 is illustrated. Long haul switching network 130 may correspond, for example, to long haul switching network 102, shown in optical communication system hierarchy 100 of FIG. 1A. Long haul switching network 130 may include nodes—such as nodes 108—that communicate using optical fiber links—such as links 110—between the nodes. Links—such as links 110—typically connect the nodes—such as nodes 108—in loops. For example, nodes 132, 134, and 136 are shown in FIG. 1B connected in a loop by links 131, 133, and 135. Link 131 connects node 136 with node 132; link 133 connects node 132 with node 134, and link 135 (shown as a broken link) would ordinarily connect node 134 with node 136. Links—such as links 131, 133, and 135—may comprise multiple optical fibers that may be used as working, protection, add, or drop fibers, in any combination, as known in the art, for transmitting signal light beams between nodes—such as nodes 132, 134, and 136. For example, communication between node 136 and node 134 would ordinarily be transmitted over working fibers of link 135. If link 135 should become disabled, illustrated in FIG. 1B by a break in link 135, communication can be rerouted for example, over links 131 and 133 between node 136 and node 134 via node 132, using protection fibers included in links 131 and 133. Such rerouting can be accomplished, as known in the art, by means of optical cross connect switches or protection switches, which may be optical cross connect switches configured to perform such rerouting.
Referring now to FIG. 1C, examples of several types of connections to a metro telecommunications switching network 140 is illustrated. Metro telecommunications switching network 140 may correspond, for example, to metro telecommunications switching network 104 or metro telecommunications switching network 106, shown in optical communication system hierarchy 100 of FIG. 1A. Metro telecommunications switching network 140 may include nodes—such as nodes 142, 144, 146, and 148—connected in a loop by links 141, 143, 145, and 147, where link 141 connects node 148 with node 142; link 143 connects node 142 with node 144, and so forth, as shown in FIG. 1C. Links—such as links 141, 143, 145, and 147—may comprise multiple optical fibers that may be used as working, protection, add, or drop fibers, in any combination, as known in the art, for transmitting signal light beams between nodes—such as nodes 142, 144, 146, and 148. Each of nodes 142, 144, 146, and 148—as well as nodes 108, shown in FIGS. 1A, 1B, and 1C, may comprise one or more optical cross connect switches. Each cross-connect switch may be configured to act as a non-blocking cross-connect switch, protection switch, add/drop module, or mux/demux, as known in the art.
Individual clients are typically connected into a metro telecommunications switching network—such as metro telecommunications switching network 140—using an add/drop module. For example, add/drop module 150 may be used, as known in the art and shown in FIG. 1C, to connect LAN 152, ATM switch 154, and access router 156 to node 142 of metro telecommunications switching network 140. Also, for example, mux/demux 158 may be used, as known in the art and shown in FIG. 1C, to connect SONET add/drop multiplexer (ADM) 160, ESCON node 162, and enterprise frame relay router 164 to node 109 of metro telecommunications switching network 140. Also, for example, SONET distributed communication system (DCS) 166 may be connected, as known in the art and shown in FIG. 1C, to node 144 of metro telecommunications switching network 140. Each node—such as node 144—of metro telecommunications switching network 140 may appropriately route the signals connected to the node using optical cross-connect switches included in the node and configured—for example, as non-blocking cross-connect switch, protection switch, add/drop module, or mux/demux—to perform the appropriate function. Thus, the cross-connect switch has come to be a fundamental component of telecommunication systems.
An optical cross-connect switch may allow light to be routed between optical fibers in such a way that any optical fiber from one side of the switch can be optically connected to any of the optical fibers on another side of the switch. Metro and long haul services may be provided using dense wavelength division multiplexing (WDM or DWDM). DWDM is a technology that uses multiple lasers and transmits several wavelengths of light simultaneously over a single optical fiber. Each signal travels within its unique color band, which is modulated by the data (text, voice, video, for example). DWDM enables the existing fiber infrastructure of the telephone companies and other carriers to be dramatically increased. DWDM systems exist that can support more than 150 wavelengths. Such systems can provide more than 1,000 Gbps of data transmission on one optical fiber. Several key components in optical communications networks—including optical add/drop modules (OADM), protection switches, and cross-connect switches—may be implemented using optical switches
Conventional fiber optical switches that connect optical fiber lines are electro-optical. Such conventional switches convert photons from the input side to electrons internally in order to do the signal switching electronically and then convert back to photons on the output side, thus being referred to as optical-electrical-optical (OEO) switches. By way of contrast, an all-optical fiber optical switch, referred to as optical-optical-optical (OOO), is a switching device that maintains the signal as light from input to output. Although some vendors call electro-optical switches “optical switches,” true optical switches, i.e., all-optical switches, support all transmission speeds. Unlike electronic switches, which are tied to specific data rates and protocols, all-optical, or OOO, switches direct the incoming data bit stream to the output port no matter what the line speed or protocol (such as IP, ATM, or SONET) and do not have to be upgraded for any changes to the protocol.
An optical switch is a device that can be used to switch a beam of light by either leaving the light path to pass through a location unaffected or changing the light path to a different direction at the location. The switching can be done mechanically, for example, by moving a mirror between two distinct and stable positions—in the path of the light, and out of the path of the light. Switching by changing a light path between two distinct and stable positions may be referred to as digital switching. Digital switching is usually implemented by a switch in which the ends of all of the optical fibers connected to the switch are in the same plane, referred to as being 2-dimensional.
For example, a 2-dimensional optical cross-connect switch can be implemented with a planar array of mirrors that can be moved into and out of the path of the light for switching light beams between optical fibers. Switching can also be done mechanically, for example, by moving a mirror continuously from one position to another in order to redirect a light path from one destination to another, which may be referred to as analog switching. Because the mirror is continuously adjustable in analog switching, the geometrical configuration in which optical fibers are connected to the switch is less constrained. For example, the ends of all of the optical fibers connected to the analog switch need not be in the same plane, so that the analog switch may be referred to as being 3-dimensional.
FIG. 2A illustrates an example of a 3-dimensional optical switch 170, as known in the art. Optical switch 170 may comprise an input fiber array 172 of input optical fibers 174. The light beam 175 from each input optical fiber 174 may be focused by a collimating lens 176, included in lens array 178, at an adjustable mirror 179, included in micro-electro-mechanical systems (MEMS) adjustable mirror array 180, where each input optical fiber 174 has a particular collimating lens 176 from lens array 178 and a particular adjustable mirror 179 from MEMS adjustable mirror array 180 dedicated to the input optical fiber 174.
Similarly, optical switch 170 may comprise an output fiber array 182 of output optical fibers 184. The light beam 185 to each output optical fiber 184 may be focused by a collimating lens 186, included in lens array 188, at an adjustable mirror 189, included in MEMS adjustable mirror array 190, where each output optical fiber 184 has a particular collimating lens 186 from lens array 188 and a particular adjustable mirror 189 from MEMS adjustable mirror array 190 dedicated to the output optical fiber 184. (It should be noted that because light can propagate in either direction along an optical fiber, the terms “input” and “output” are used for convenience and do not necessarily limit the direction of signal propagation.) Thus, there is a dedicated adjustable mirror for each input and each output optical fiber of optical switch 170.
FIG. 2B shows an example of a single adjustable mirror—such as adjustable mirror 179—from MEMS adjustable mirror array 180, of a typical silicon-on-insulator (SOI) construction, as known in the art. Adjustable mirror 179 is shown mounted in gimbals 191, which may also act as a spring for returning adjustable mirror 179 to a neutral position, as known in the art.
FIG. 2C shows adjustable mirror 179 in cross section before selective etching of silicon dioxide (SiO2) material 193 is used to form the components of the gimbals 191 and electrodes 192, and FIG. 2D shows adjustable mirror 179 in cross section after etching is used to form the components of the gimbals 191 and electrodes 192. For the particular example illustrated in FIGS. 2B-2D, the position, i.e., angle, of adjustable mirror 179 may be controlled by an electric field applied at electrodes 192, as known in the art. Alternative configurations may control the position of the adjustable mirror using magnetic fields, as known in the art. A MEMS adjustable mirror array has been manufactured by Lucent Technologies, Inc. under the trade name “Microstar® Mems Mirrors”. An alternative to an array of adjustable mirrors—such as MEMS adjustable mirror arrays 180 and 190—may be a spatial light modulator (SLM), such as that disclosed by U.S. Pat. No. 6,430,328 issued to Culver, et al., which could be used to steer light beams 175 and 185 in place of MEMS adjustable mirror arrays 180 and 190.
Mirror positioning for the 3-dimensional analog optical switch requires a high degree of accuracy in order to direct a light beam from any one of an input array of optical fibers to any chosen one of an output array of optical fibers, also referred to as “targeting”. U.S. Pat. No. 6,101,299 issued to Laor discloses a fiber optical control system for use in an optical switch in which a feedback control system collects a feedback signal from an output fiber end by incorporating a sensor for detecting the feedback signal in front of the collimating lens for the fiber for targeting the beam. The limited targeting accuracy of the configuration limits applicability of the feedback control system to direct fiber-fiber or fiber-mirror-fiber configurations. Thus, the system disclosed by Laor is impractical for typical 3-dimensional analog switches requiring more than two mirrors in the optical path.
U.S. Pat. No. 5,206,497 issued to Lee discloses a fiber optical control system for use in an optical switch in which a partially silvered mirror is used to separate components of a light beam so that a monitor component reflected off the mirror can be used for aligning the beam, while a reduced intensity signal-carrying, or payload, component is transmitted through the mirror to the output array of optical fibers. The transmitted (payload) and monitor components have the same wavelength.
As can be seen, there is a need for an analog optical switch and control system that achieves accurate beam alignment for multiple mirror switch configurations. Also, there is a need for an optical switch that can obtain accurate beam alignment without sacrificing signal intensity.